1. Field of the Invention
The present invention relates to a method for dissolving hard-to-dissolve nuclear fuels such as thorium dioxide and plutonium dioxide, in a nitric-acid, fluoride ion-containing solution.
2. Background of the Invention
If plutonium-containing fuel assemblies are employed, Pu is used as sintered oxide PuO.sub.2. In this form, it is particularly well suited as fuel because of its high melting point and the high sintering density. However, these advantages turn into disadvantages if the spent nuclear fuel is to be removed periodically from the reactor for the purpose of separating the fissionable material and is to be reprocessed. Due to the thermal history of the plutonium, it has extremely little solubility in the solvent, nitric acid, which is perferred for fuel processing. This applies especially to U/Pu-fuels which are mechanically mixed in their manufacture. These contain separate UO.sub.2 - and PuO.sub.2 -phases, of which only the UO.sub.2 -phases are soluble in nitric acid, and thus mainly PuO.sub.2 remains as an insoluble residue in the solution.
The problem of dissolved PuO.sub.2 is dealt with in German Published Non-Prosecuted Application No. 26 19 089. As solvent, a mixture of nitric acid and hydrofluoric acid is described. However, it is an obstacle to the practical employment of this method that this mixture has an extremely corrosive effect on metallic dissolving vessels. Even if special steels and rare metals are used (except platinum), the corrosion rates are so high that an extended use of metal vessels is not possible, especially since as undesirable fringe phenomena, contamination of the nuclear fuel solution by dissolved metal would occur. When dissolving small amounts of PuO.sub.2 on a laboratory scale, this problem can be circumvented through the use of fluorine-containing, corrosion-resistant plastics, for instance, polytetrafluoroethylene. To apply this vessel technique to the dimensions of a dissolution vessel in a reprocessing plant is extremely difficult.